Fix NuGet badge on README to show pre-release (#3189)

Author: eerhardt

README.md

@@ -14,7 +14,7 @@ Along with these ML capabilities, this first release of ML.NET also brings the f
 
 ## Installation
 
-[![NuGet Status](https://img.shields.io/nuget/v/Microsoft.ML.svg?style=flat)](https://www.nuget.org/packages/Microsoft.ML/)
+[![NuGet Status](https://img.shields.io/nuget/vpre/Microsoft.ML.svg?style=flat)](https://www.nuget.org/packages/Microsoft.ML/)
 
 ML.NET runs on Windows, Linux, and macOS using [.NET Core](https://github.com/dotnet/core), or Windows using .NET Framework. 64 bit is supported on all platforms. 32 bit is supported on Windows, except for TensorFlow, LightGBM, and ONNX related functionality.